This invention relates to heat and moisture exchangers and breathing circuits including a heat and moisture exchanger.
Heat and moisture exchangers (HMEs) are used to warm and moisten gas supplied to a patient. The HME comprises a housing coupled in the patient breathing circuit, through which both inhaled and exhaled gas pass. Within the housing, an exchange element takes up a part of the heat and moisture in the expired gas and transfers a part of this to the inspired gas when flow is reversed. The exchange element may be a coiled strip of corrugated paper treated with a hygroscopic material or a foam. HMEs are sold by Portex Limited of Hythe, England under the trade mark Thermovent. Examples of HMEs are described in: GB 2277689; GB 2268496; GB2267840; EP 535016; EP 533644; EP 387220; EP 265163; EP 413127; U.S. Pat. No. 4,516,573; U.S. Pat. No. 4,090,513; U.S. Pat. No. 4,771,770; U.S. Pat. No. 4,200,094; and U.S. Pat. No. 4,048,993.
HMEs have the advantage over conventional humidifiers in that they can be compact, light, and disposable after a single use. The HME should preferably have a low resistance to flow, a high efficiency in exchange of heat and moisture, a long life and be easily made at low cost. The HME must also present no hazard to the patient.